Integrins are a class of widely distributed cell surface adhesion receptor membrane proteins that are essential for normal vertebrate development. These proteins play a key role in a variety of physiological processes including thrombosis, inflammation, restenosis, and malignant transformation (Schwartz, M. A. et al. 1995 Ann. Rev. Cell Dev. Biol. 11:549-599; Hynes, R. O. 1992 Cell 69:11-25). Accordingly, modulation or regulation of integrin function has been proposed as a strategy for development of numerous therapeutics (Schwartz, M. A. et al. 1995 Ann. Rev. Cell Dev. Biol. 11:549-599; Hynes, R. O. 1992 Cell 69:11-25).
Integrin proteins are heterodimeric and consist of two polypeptide subunits, .alpha. and .beta., that pair to form more than 20 distinct receptors (e.g., .alpha.IIb/.beta.3, .alpha.1/.beta.1, .alpha.2/.beta.1, etc.; Hynes, R. O. 1992 Cell 69:11-25). These receptors generally consist of a large extracellular domain formed by the .alpha. and .beta. subunits, a transmembrane segment also from Each subunit, and two short cytoplasmic C-terminal tails (Schwartz, M. A. et al. 1995 Ann. Rev. Cell Dev. Biol. 11:549-599). The extracellular domains bind to proteins or other receptors in the extracellular matrix while the cytoplasmic domain forms a link to the cytoplasmic skeleton.
These receptor proteins can transmit information in both directions across the plasma membrane. As a result, integrins may transduce signals within cells to initiate cellular processes such as division, secretion, or gene expression. An important feature of these proteins is that they often must undergo activation in order to initiate cellular activity (Hynes, R. O. et al. 1992 Cell 69:11-25). Stimuli for activation may include soluble mediators such as hormones and cytokines as well as solid-phase reactants such as the extracellular matrix of another cell. Integrins are activated at the appropriate time and place in the cell through input by specific physiological signals such as thrombogenic agonists, antigen, cytokines, or T cells (Hynes, R. O. et al. 1992 Cell 69:11-25). Similarly, inactivation of integrins is also important to maintenance of homeostasis in mammalian systems.
Methods for identifying binding partners of integrin function include the yeast two hybrid cloning system and affinity chromatography (Shattil, S. J. et al. 1995 J. Cell. Biol. 131:807-816; Hannigan, G. E. et al. 1996 Nature 379:91-95; Kolanus, W. et al. Cell 86:233-242). However, there is a need for methods and systems for identifying regulators which activate integrins in mammalian cells.